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MF10ACN Datasheet(PDF) 21 Page - Texas Instruments

Part No. MF10ACN
Description  Universal Monolithic Dual Switched Capacitor Filter
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
Logo TI1 - Texas Instruments

MF10ACN Datasheet(HTML) 21 Page - Texas Instruments

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3.0 Applications Information
The MF10 is a general-purpose dual second-order state
variable filter whose center frequency is proportional to the
frequency of the square wave applied to the clock input
(f
CLK). By connecting pin 12 to the appropriate DC voltage,
the filter center frequency f
O can be made equal to either
f
CLK/100 or fCLK/50. fO can be very accurately set (within
±6%) by using a crystal clock oscillator, or can be easily
varied over a wide frequency range by adjusting the clock
frequency. If desired, the f
CLK/fO ratio can be altered by
external resistors as in
Figures 9, 10, 11, 13, 14, 15. The
filter Q and gain are determined by external resistors.
All of the five second-order filter types can be built using
either section of the MF10. These are illustrated in
Figure 1
through
Figure 5 along with their transfer functions and some
related equations.
Figure 6 shows the effect of Q on the
shapes of these curves. When filter orders greater than two
are desired, two or more MF10 sections can be cascaded.
3.1 DESIGN EXAMPLE
In order to design a second-order filter section using the
MF10, we must define the necessary values of three param-
eters: f
0, the filter section’s center frequency; H0, the pass-
band gain; and the filter’s Q. These are determined by the
characteristics required of the filter being designed.
As an example, let’s assume that a system requires a
fourth-order Chebyshev low-pass filter with 1 dB ripple, unity
gain at DC, and 1000 Hz cutoff frequency. As the system
order is four, it is realizable using both second-order sections
of an MF10. Many filter design texts include tables that list
the characteristics (f
O and Q) of each of the second-order
filter sections needed to synthesize a given higher-order
filter. For the Chebyshev filter defined above, such a table
yields the following characteristics:
f
0A = 529 Hz QA = 0.785
f
0B = 993 Hz QB = 3.559
For unity gain at DC, we also specify:
H
0A =1
H
0B =1
The desired clock-to-cutoff-frequency ratio for the overall
filter of this example is 100 and a 100 kHz clock signal is
available. Note that the required center frequencies for the
two second-order sections will not be obtainable with
clock-to-center-frequency ratios of 50 or 100. It will be nec-
essary to adjust
externally. From
Table 1, we see that Mode 3 can be used to
produce a low-pass filter with resistor-adjustable center fre-
quency.
In most filter designs involving multiple second-order stages,
it is best to place the stages with lower Q values ahead of
stages with higher Q, especially when the higher Q is greater
than 0.707. This is due to the higher relative gain at the
center frequency of a higher-Q stage. Placing a stage with
lower Q ahead of a higher-Q stage will provide some attenu-
ation at the center frequency and thus help avoid clipping of
signals near this frequency. For this example, stage A has
the lower Q (0.785) so it will be placed ahead of the other
stage.
For the first section, we begin the design by choosing a
convenient value for the input resistance: R
1A = 20k. The
absolute value of the passband gain H
OLPA is made equal to
1 by choosing R
4A such that: R4A =−HOLPAR1A =R1A = 20k.
If the 50/100/CL pin is connected to mid-supply for nominal
100:1 clock-to-center-frequency ratio, we find R
2A by:
The resistors for the second section are found in a similar
fashion:
The complete circuit is shown in
Figure 16 for split ±5V
power supplies. Supply bypass capacitors are highly
recommended.
www.national.com
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